Process for manufacturing porous cast iron



United States Patent PROCESS FOR MANUFACTURING POROUS CAST IRON 8020Kawasaki, Minatoku, Tokyo, Japan No Drawing. Application March 26,.1953, Serial No. 344,891

6 Claims. (Cl. 148-417) This invention relates to a method for easilyproducing cast iron which is excellently permeable by oil and issuitable for use in oilless bearings or the like.

This invention has for its object to increase the porosity of oilpermeable cast iron so that it may contain the maximum possible amountof oil, without sacrificing its mechanical strength.

Where it is desired to make a bearing of an oil perme able metallicmaterial made by sintering finely divided material in a forming moldtherefor, the porosity of the surface of the product thus obtained tendsto be im paired by the subsequent machining operations. This difficultymanifests itself particularly when such metals as copper, tin, etc., areused as the raw material.

it is well known that if a cast iron is repeatedly subjected toannealing, the phenomenon of the expansion of the volume of the castiron i. e. the phenomenon of the so-called growth of the cast iron takesplace. Numerous investigations have been made to prevent such a growth.This applicant noticed that this expansion Percent Carbon (in total) u3.20 Combined carbon 0.50 Silicon 2.00 Sulphur 0.07 Phosphorus 0.70Manganese 0.80

was heated to 700 C. and maintained at this temperature for about threehours, and thereafter it was cooled slowly to room temperature. Thisheat treatment procedure was repeated thirty times. The resultingproduct.

was a porous material having a specific gravity of 5.7. It was thenimmersed into a lubricating oil bath heated to 120 (3.. whereby a rawcast iron containing 26% of lubricating oil by volume was obtained. Thebearing made from this final product by machining proved to be a verysatisfactory one as a main bearing of a grinding machine with a mainshaft revolving at a speed of 1500 R. P. M., the porosity of themachined bearing surface thereof being seemingly not impaired at all.Indeed, oilless bearings made of this oil permeable cast iron ob tainedfrom grey cast iron as set forth previously have been used in practicewith a favourable comment.

Since it seems that the formation of iron oxides around graphite-flakesdistributed in the mass of a cast iron subjected to repeated annealingis one of causes of the growth of cast iron, this applicant arrived atthe conclusion that the porosity of cast iron might be increased,

Copper 2,763,583 Patented Sept. 18, 1956 if such iron oxides wereremoved therefrom. Thus, the same raw grey cast iron as used in theprevious experiments was heated to 700 C., maintained at thattemperature for about three hours and thereafter cooled slowly toordinary temperature. fifty times. The porous cast iron thus obtained,which had a specific gravity of 5.506, was immersed in a bath of 0.25%sulphuric acid. solution at room temperature for about 24 hours, andthen thoroughly washed with water and completely dried. Itsspecificgravity then was 5.439. When a circulardisc made of this product 30.2mm. in. diameter and 10.2 mm. in thickness was immersed in. a bath. oflubricant, the quantity of lubricant impregnated in it was 1.666 gr.,whereas the quantity of lubricant impregnatedin a disc having: the samesize, but not subjected to said acid pickling treatment proved to beonly 1.00 gr. It is thus ascertained that the pickling treatment doesincrease the porosity of cast iron which confirms the truth of theapplicants aforementioned conclusion. The results of this investigationhave already been published.

Through a series of investigations just mentioned, this applicanthasrecently succeeded in obtaining excellent results in that an oilpermeable cast iron having a. still much. greater mechanical strengthcan be obtained, and the experimental investigation in the direction ofdefinitely increasing the porosity of the material without appreciablydecreasing its mechanical strength has been advanced.

The latest investigations of the oilless bearing, the mostimportantintended field of use of oil permeable cast iron, tends to bedirected. towards the enhancement ofthe mechanicalstrength of thelatter, in order to meet the requirements of: large type or heavy dutybearings. Consequently, this applicant has further carried out a similarexperimental investigation with ferrous materials other than grey castiron for the purpose of the enhancement of their mechanical strengths.He has found out that when a white pig iron is annealed so as to producea black heart malleablecasting, the structure of the latter will becomesuch that its free carbon is: distributed in the. matrix of. the iron,and that when such a malleable casting is repeatedly heated and cooled,there will be obtained a porous material which is more homogenous thangrey cast iron and its mechanical strength is surprisingly, increased.Thus a white pig iron (tensile strength 29.8 kg./mm.. Brinell hardness315 and specificgravity 7.69), having the composition of:

Per cent Carbon c 2.48 Silicon. 0.92- Manganese 0.34 Phosphorus 0.11Sulphur M..- 0.09 Chromium 0.03

i and, having been annealed atabout 850 Crfor twentysix hours, washeated to a temperature somewhat above or below the A1 transformationpoint, that is, 800 C., and then cooled to atmospheric temperaturerepeatedly, the number of repetitions being fifty times, whereby itsmechanical properties became such that its tensile strength wa 27.2kg/mmfi, Brinell hardness 80, and specific gravity 7.1. Uponimpregnating the material thus obtained with lubricant, it was foundthat it could contain 4% by volume of the lubricant. Since with greycast iron after the growth the mechanical strength is at most 13-15kg./mm. it is apparent from these results that with white pig ironhaving a mechanical strength of the order of 27 kg./mm. a mechanicalstrength of at least twice that of grey cast iron is readily obtained.

Promoting the investigation for the purpose of provid- This procedurewas repeated ing a porous, oil permeable cast iron material having astill greater mechanical strength in this way, this applicant has foundout that a cast iron which has been caused to have a spheroidalgraphitic structure has a greater tensile strength than usual. Inconsequence thereof, this applicant has succeeded, by heating andcooling such cast iron above and below its A1 transformation pointrepeatedly to effect the growth thereof, to obtain a porous, oilpermeable cast iron having a mechanical strength three to four times asgreat as that of an ordinary grey cast iron, and yet capable ofabsorbing substantially the same amount of lubricant as that absorbed bythe latter. In this experiment, a nodular cast iron (tensile strength60.4 kg./mm. Vickers hardness number 290, specific,

gravity 7.3) containing 0.3% magnesium which was added thereto byemploying 80:20 nickel-magnesium alloy and 0.5 silicon added thereto asan inoculant, was repeatedly heated and cooled between a temperaturesomewhat above or below 800 C. and ordinary temperature, the number ofrepetition being fifty times, thereby producing a grown up materialhaving a tensile strength of 54 kg./mm. Vickers hardness number of 180,and specific gravity of 6.6, which grown up material when subjected toanoil impregnating treatment was capable of absorbing 6% by volume (0.75%by weight) of lubricant. This material is found to be suitable for usein heavy duty oilless bearings.

. Based on the results of the aforementioned experiment, and havingascertained that the degree of porosity of a cast iron can be enhancedby repeatedly annealing it to produce a very porous cast iron and thenimmersing this porous cast iron into a bath of sulphuric acid solutionso as to eliminate the iron oxides distributed in that cast iron, thisapplicant hit on the idea that it would be possible to increase theporosity of a cast iron without sacrificing its mechanical strength, ifthe cast iron could be converted into one having a small specificgravity by repeatedly subjecting it to annealing operations to produce,in the first place, a porous cast iron having a great amount of ironoxides formed therein, and then reducing said iron oxides to metalliciron. Thus, a porous grey cast iron material (a rod of 30 mm. diameter,200 mm. long, specific gravity 5.7) obtained from the previousexperimental investigation but not yet subjected to said oilimpregnating treatment, was exposed to air at ordinary temperature forsixty days. As it was observed that reddish rust of iron oxides had beenformed throughout the mass of this material, the latter was subjected tothe reduction treatment in a reducing gas stream at a temperature of 700C. to 800 C. made from cokes. Then it was found that .these iron oxides.had been definitely re duced and the porosity greatly increased. Itthus became apparent from this experiment thatprior to the reductiontreatment according to the procedure of this invention, the oilcontaining capacity of the cast iron material was 0.5% by weight,whereas that-of the same cast iron after said treatment was 2.5% byweight, that is, its oil containing capacity wa multiplied five times.

Furthermore, the results of tests of the porous cast iron I claim:

1. The method of manufacturing porous cast iron comprising repeatedlyheating cast iron to the temperature of its A1 transformation point andcooling the cast iron to the ambient room temperature, exposing theporous product so obtained to the oxidizing action of air at the ambientroom temperature for an extended period of time, and reducing the oxidesof the product in a stream of reducing gas.

2. The method according to claim 1 in which the heating and cooling isrepeated fifty times from the high temperature of substantially 800 C.to the low temperature of the ambient room atmosphere, the exposure isfor a time of the order of sixty days, and the reduction is at atemperature in the range from 700 to 800 C.

3. The method of manufacturing oil impregnated grey cast iron comprisingheating the grey cast iron to about 700 C., maintaining the heating forsubstantially three hours, cooling the grey cast iron slowly to roomtemperature, repeating said heating, maintaining and cooling at leastthirty times, exposing the iron to the ambient atmosphere andtemperature for a period of substantially sixty days, subjecting theiron to an oxide reducing gas stream at a temperature in the range offrom 700 to 800 C., and immersing the iron in lubricating oil untilsubstantially no further oil is absorbed by the iron.

4. The method according to claim 3 in which the repetition of theheating, maintaining and cooling steps is of the order of fifty times.

5. The method of manufacturing oil impregnated white pig iron comprisingheating the white pig iron to a temperature substantially equal to thatof its A1 transformation point, cooling the white pig iron toatmospheric temperature, repeating the said heating and cooling fiftytimes, exposing the iron to the ambient atmosphere and temperature foran extended period, subjecting the iron to an oxide reducing gas stream,and impregnating the iron with oil.

6. The method of manufacturing oil impregnated nodular cast ironcontaining 0.3% magnesium and 0.5 silicon comprising heating the nodularcast iron to a temperature of substantially 800 C., cooling the nodularcast iron to room temperature, repeating said heating and coolingsubstantially fifty times, exposing the nodular cast iron to theoxidizing action of the atmosphere at ordinary temperature for anextended period, subjecting the repeatedly heated and cooled, andexposed nodular cast iron to an oxide reducing gas stream, and then impregnating the nodular cast iron with oil.

References Cited in the file of this patent UNITED STATES PATENTS1,511,063 Pack Oct. 7, 19 24 Lenel Ian. 16, 1940 OTHER REFERENCES Steeland Its Heat Treatment by Bullens, vol. 2, 5th ed., 1948, page 255.

1. THE METHOD OF MANUFACTURING POROUS CAST IRON COMPRISING REPEATEDLYHEATING CAST IRON TO THE TEMPERATURE OF ITS A1 TRANSFORMATION POINT ANDCOOLING THE CAST IRON TO THE AMBIENT ROOM TEMPERATURE, EXPOSING THEPOROUS PRODUCT SO OBTAINED TO THE OXIDIZING ACTION OF AIR AT THE AMBIENTROOM TEMPERATURE FOR AN EXTENDED PERIOD OF TIME, AND REDUCING THE OXIDESOF THE PRODUCT IN A STREAM OF REDUCING GAS.